“If you regularly experience a rapid or irregular heartbeat, you may be experiencing atrial fibrillation. Atrial fibrillation is a condition that occurs when the upper chambers of your heart receive irregular electrical signals. The chamber responds by quivering, which in turn overloads the AV node between the atria and the ventricles. The result is the irregular heartbeat called atrial fibrillation.”

alot.com “Understanding Atrial Fibrillation”